Method and apparatus for supporting mobile communications in asynchronous transfer mode based networks

ABSTRACT

Distributed call setup and rerouting are realized in a mobile-communications network. A connection tree is set up within the network, e.g., upon a mobile user accessing a base station. The connection tree comprises communication routes from a fixed point in the network, the root of the tree, to each base station within a vicinity of the base station accessed by the mobile user. When the mobile user moves from one cell to another within the connection tree, the call is rerouted to another route within the connection tree.

The United States Government has certain rights to this invention underContract CDR-881111 awarded by the National Science Foundation.

This application is a division of application Ser. No. 08/067,717, filedon May 26, 1993.

BACKGROUND OF THE INVENTION

This invention relates generally to mobile communications systems and,more specifically, to distributed call setup, admission, control andrerouting in mobile communications systems capable of supportingasynchronous transfer mode.

In recent years, interest has rapidly grown in two distinct areas of thecommunications field. First, the popularity of mobile, or personal,communications has increased immensely and is expected to grow in thenear future to the point where existing systems will be unable tosupport the demand for mobile communications. The basic problem facingthe future of mobile communications systems is the lack of availablebandwidth for the mobile user's wireless transmission to a fixednetwork.

Current mobile communications systems employ the concept of "cells." Acell is a geographical area which is assigned to a corresponding basestation which is in turn wired into a fixed communications network. Thecapacity of a cellular system can become quite high since the availablebandwidths can be reused among the various cells. The fixed network istypically a "mesh network." A mesh network is comprised of numerousswitches connected together by communication links. The mesh network isset up so that a communications route may be traced from any one switchto any other in the network through at least one, and more often, manycombinations of links and switches. Some of the switches in the network,in addition to being connected to other switches, will also be connectedvia communication links to one or more of the base stations and/or fixedtermination points such as a home telephone.

When a mobile user wishes to place a call, the call is transmittedthrough a communication medium, often a radio channel, to the basestation assigned to the user's cell. From the base station, the call iscarried by the mesh network to the user's intended destination. When themobile user moves from one cell to another, a call handoff, or handover,between base stations takes place. The handoffs are performed by acentral system controller. This is known as centralized call processing.

Due to the limited bandwidth available for the wireless transmissions ofmobile users' calls, each cell can handle only a limited number ofcallers. Thus, to meet the needs of an increasing population of mobileusers, the area assigned to each cell must be decreased. Therefore, thenumber of such cells, and their corresponding base stations, increasesfor a given geographical area. The smaller cell size means that the rateof call handoffs per second in a mobile communications system increases,thereby placing a great burden on the central system controller. It ispredicted that in the near future, the centralized system controllers oftoday's systems will be unable to handle the call handoff rate whichwill be necessary to meet the demand for mobile communications. The needfor a distributed call processing system, rather than a centralizedsystem, has been identified, but no comprehensive solutions have as yetbeen found.

The second area of communications which has been the focus of muchresearch is the standardization of broadband integrated services digitalnetworks (B-ISDN) utilizing the asynchronous transfer mode (ATM).Present communications systems rely on circuit switching techniques. Inthese circuit switching systems, a circuit path between the caller andhis destination is found and used exclusively for a single call untilthe call terminates. On the other hand, future B-ISDN systems willemploy packet, or cell, transport techniques. This means, essentially,that a communication is broken up into discrete "packets", or "cells"(unrelated to the geographic cells of mobile communications systems),which are sent one at a time through the system and received at theintended destination as an uninterrupted communication. Packets frommany different callers may simultaneously share the same communicationlink, thus making these B-ISDN systems a more efficient means ofcommunication than the circuit switching systems.

ATM is the target mode for future B-ISDN systems. The other, and at onetime favored, choice for B-ISDN packet communications was synchronoustransfer mode (STM) which would handle the packets from a givencommunication during allocated time slots occurring on a regular basis.For reasons unimportant to this invention, ATM was chosen over STM.

The ATM packets are themselves divided into two sets of information. Oneset is the information which the user intends to transmit and the otherset is called the "header." The header contains routing information,including a virtual channel identifier (VCI). The VCI, simply put, is acode assigned to the packet which lets an ATM switch know where to sendthe packet next, based on the switch port where the packet has beenreceived.

A conventional ATM B-ISDN switch, hereinafter called an ATM switch, hasseveral input and output ports. Embodied in the implementation of aconventional ATM switch is a "lookup table." The lookup table may bethought of as having four columns: input port, incoming VCI, outputport, and outgoing VCI. For every possible input port and incoming VCIcombination, there is a corresponding output port and outgoing VCIcombination programmed into the lookup table. When an ATM switchreceives a packet at a given port, the ATM switch will find the row inthe lookup table which has the incoming VCI and input port whichcorrespond to that of the received packet. The ATM switch will thenswitch or route the packet to the output port which appears in the samerow and replace the incoming VCI in the packet header with the outgoingVCI. This process is performed at each switch until the packet arrivesat its destination.

The heightened interests in both mobile communications and B-ISDNcommunication using ATM which has been described above has led to thepresent invention. Up until now, no system, existing or proposed, hasprovided a technique for distributed, as opposed to centralized, callsetup and rerouting in a mobile ATM based B-ISDN system with several ATMswitches.

SUMMARY OF THE INVENTION

In mobile communications, distributed call setup and rerouting arerealized. A communication route is determined which includes aconnection from a base station via a switching node. A connection treeis determined, to include potential connections from other base stationsto the switching node. When the mobile user moves from one cell toanother, corresponding to a change in base station within the connectiontree, the call is automatically handed over or rerouted to another routewithin the connection tree. By obviating the need for a centralprocessor for handovers within the connection tree, a greater number ofcommunications may be supported than in conventional mobilecommunications systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the invention will becomeapparent, and its construction and operation better understood, from thefollowing detailed description when read in conjunction with theaccompanying drawings, in which:

FIG. 1 is a representation of a connection tree in a mesh networkaccording to an exemplary embodiment of the present invention;

FIG. 2 is a representation of a simple connection tree according to apreferred first embodiment of the present invention;

FIG. 3 is a representation of an ATM Switch/VCI Translator;

FIG. 4 is a representation of a simple connection tree according to apreferred second embodiment of the present invention in which one of theATM switches is an ATM Switch/VCI Translator, and of the lookup tableswhich correspond to two of the switches;

FIG. 5 is a representation of an ATM switch/rerouter;

FIG. 6 is a representation of a simple connection tree according to apreferred third embodiment of the present invention in which there arethree ATM Switch/Rerouters, and of the lookup tables which correspond totwo of the switches; and

FIG. 7 is a representation of a simple a connection tree according to apreferred fourth embodiment of the present invention in which one of theswitches is a Switch/Rerouter, and of the lookup tables corresponding totwo of the switches.

FIG. 8 is a flow diagram of a communications method in accordance with apreferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One embodiment of the general architecture of the present invention maybe seen in FIG. 1. At call setup time, a connection tree 10 within acommunications network 12 having a packet switching architecture isdefined for a mobile user. The communications network 12 is a meshnetwork as described in the Background section hereinabove.

The connection tree 10 established within the mesh network 12 extends aconnection from a fixed point 11 of the network, called the root, toeach one of the base stations 17 in the neighborhood 20 of the mobileuser. The base station in charge of a mobile user is called the mobileuser's access point, base station 22 in this example. The neighborhood20 may be defined as the area within a predetermined distance of themobile user's access point 22. A connection tree 10 provides a route,referred to as a virtual channel connection, to the mobile user's accesspoint 22 as well as a virtual channel connection for each of itsneighboring mobile access points 17 such that a concatenation of one ofthese virtual channel connections with another virtual channelconnection 25 provides an end-to-end connection in which the mobile usercan be the source or destination of the traffic. At the time aconnection tree 10 for a mobile user is set up, a unique connection treeID is assigned to the user. The significance of the connection tree IDwill be explained later.

As discussed above, the connection tree 10 is set up with reference tothe mobile user's location. This is the case whether the mobile user isthe source or destination of a call. In both instances, the connectiontree 10 will be set up at the mobile user's first interaction with abase station 17.

Further explanation of the connection tree architecture will be madewith reference to FIG. 1. FIG. 1 represents just one of a large numberof possible connection tree configurations. Connection tree links areshown with thick lines, such as link 3, and other links of the networkare shown with narrow ones, such as link 4. For the remainder of thisdisclosure, the existence of links not included in the connection treewill be ignored. This, of course, does not mean that they should not bethought of as part of the network. Also, although they appear in thefigures, the links which are included in the connection tree will not bediscussed and should be assumed to connect the switches and basestations as shown in the figures.

Returning to FIG. 1, at the call setup time, the mobile user iscommunicating to a base station 22 connected to switching node 5.Switching nodes 7, 8, and 9 are the switches to which all theneighboring base stations 17 are connected. In any end-to-end connectionfor which the mobile user is the source or the destination, thecommunication path is divided into two parts. One part of the path iscontained within the connection tree 10. This part of the path changesas the result of the mobile user's connection hand-offs. Details of thisprocedure are described hereinbelow. The other part of the path, whichis from the root 11 of the connection tree to the other end of theconnection 18, remains fixed throughout most of the connection lifetime.It should be noted, however, that the fixed part of the path will notalways be necessary. A mobile user may wish to communicate with adestination, fixed or mobile, within the same connection tree. In thisscenario, the communications will be routed to the root of theconnection tree, but instead of then being switched out of the tree andthrough a fixed route, the communications will instead be routed backinto the tree to the intended recipient. Thus, the two-part pathconsists of two connection trees rather than a connection tree and afixed route.

Whenever the user reaches the boundary of the connection tree 10, a newconnection tree is established so that the neighboring base stations ofthe mobile user's access point belong to a new connection tree. Thisprocedure is called the connection tree hand-off. Any time there is ahand-off from one base station 17 to another, the current virtualchannel connection is disabled and the virtual channel connectionterminating at the new mobile access point is enabled. The part of thepath in the wired network originating at the mobile user's calldestination point 18 and which terminates at the root 11 of theconnection tree, denoted by 25, is fixed and reused.

An example may be illustrated referring to FIG. 2. FIG. 2 depicts asimple connection tree having three switching nodes 14, 15, 16 and fourbase stations 33, 34, 35, 36. The mobile user is represented by anautomobile 29. Upon placing a call in the vicinity of base station 34,the connection tree is set up. Switch 15 is the root of the connectiontree and four virtual channel connections 50, 51, 52, 53 to the fourbase stations 33, 34, 35, 36 are provided to the mobile user 29. At theoutset, the mobile user's access point is base station 34 and virtualchannel connection 51 is enabled. As the mobile user 29 moves out of thegeographical cell 40 assigned to base station 34 and into the cell 41assigned to base station 35, a call hand-off takes place whereby virtualchannel connection 51 is disabled, virtual channel connection 52 isenabled and base station 35 becomes the mobile user's access point.

In this embodiment of the invention, call hand-off is initiated by themobile user. This is called mobile initiated hand-off. A hand-off may beinitiated by any one of several well known means. For example, the usermay monitor power from each of the base stations in its connection treeand initiate a hand-off whenever the signal strength received from theuser's current base station approaches some threshold of unacceptabilityand there is at least one base station in the tree from which anacceptably high power level is detected. In such a case, the mobile userwould initiate a hand-off to any base station from which acceptably highpower is detected. The rerouting of the connection is done within theconnection tree and the involvement of a central network call processorfor rerouting is unnecessary. Since a distributed rerouting ofconnections is provided, a large number of hand-offs can be realized ina "micro-cell/pico-cell" environment which has a high frequency ofhand-offs.

FIG. 8 shows in simple flow chart form the steps performed in theconnection tree switching scheme as described above. A communicationsrequest is detected by a base station in block 160. This is followed byconnection tree setup and setup of the fixed portion of the end-to-endconnection in blocks 161 and 162 respectively. The specific route withinthe connection tree between the root and the base station where thecommunication was detected is defined in block 163. In block 164, thesystem looks for a change in the user's base station. If the user isstill within the vicinity of the base station, the communicationcontinues in block 165. If the user has moved into the vicinity of a newbase station, the system looks to see if the new base station is withinthe connection tree in block 166. If so, control returns to block 163and a new route within the connection tree is defined. If the new basestation is not within the connection tree, control returns to block 161and a new tree is set up. This will continue until the communication isterminated, which can occur at any time.

Two schemes for rerouting of the connections in the connection tree willbe discussed herein. The schemes are devised for a packet switchedB-ISDN asynchronous transfer mode (ATM) system as described in theBackground section of this specification. This should not be interpretedto limit the use of the connection tree system discussed above to apacket switched B-ISDN ATM system. It is possible to use the connectiontree architecture described above in other types of packet switchedcommunications systems.

The term "switch" used in the description of these schemes should beinterpreted to include any switching node or network, such as a localarea network (LAN) or metropolitan area network (MAN), which is capableof ATM packet transport based on virtual channel identifiers. The terms"communication" or "call" as used in the description should beinterpreted to include phone, fax, and any other type of communicationwhich can be supported by a B-ISDN ATM system.

A. Scheme 1

As described above, at call setup time, a connection tree is defined forthe user. In this embodiment of a connection tree, the root of the treeis a switch with an added feature which is described herein. This switchis shown in FIG. 3 and is called an ATM Switch/VCI Translator.

An ATM Switch/VCI Translator performs the virtual channel connectionrerouting in the mobile user's connection tree. It is an ATM switchhaving input ports 55 and output ports 56 with additional componentswhich will be discussed with reference to FIG. 3. At each input port 55of the switch, there is a VCI monitor/comparator 57 which has read/writeaccess to the switch's routing lookup table 58. The concept of a routinglookup table was discussed in the Background section of thisspecification.

The VCI monitor/comparator 57 copies all incoming packet headers, eachof which includes a virtual channel identifier, to its local memory andcompares them to the lookup table 58. It associates a virtual channelconnection from the root of the tree to the base station of a mobileconnection with an incoming and outgoing VCI as well as the incoming andoutgoing switch ports of the connection. To clarify this procedure anexample is given in FIG. 4.

FIG. 4 shows a simple connection tree. The root of the connection treeis an ATM Switch/VCI Translator 75 having three input ports 59, 62, 73and three output ports 60, 61, 74. The other switches 76, 77 areordinary ATM switches. Switch 76, through which our call will travel,has three input ports, 63, 66, 68 and three output ports 64, 65, 67.Portions of the lookup tables of switches 75 and 76 are also shown inFIG. 4. The lookup table of an ATM Switch/VCI Translator differs fromthat of a conventional ATM switch in that only certain rows will beenabled depending on which of the base stations within the tree isserving as the mobile user's access point.

In FIG. 4, when the mobile user is communicating to the base station 78,there is a virtual channel connection 85 established between the root 75of the connection tree and the base station 78. Communications leavingbase station 78 travelling along path 85 leave base station 78 withvirtual channel identifier VC1. The packets are transmitted via outputport 70 into input port 66 of switch 76. Switch 76 reads VC1, consultsthe third row of its lookup table and assigns virtual channel identifierVC3 to the packet to let ATM Switch/VCI Translator 75 know where thecommunication is coming from and what the packet's final destination is.The packet is transmitted via output port 64 of switch 76 and entersinput port 62 of ATM Switch/VCI Translator 75. When the VCIMonitor/Comparator at input port 62 reads VC3, the ATM Switch/VCITranslator enables rows one and three which correspond to path 85. Thepacket is then assigned virtual channel identifier VC4, as per row threeof the lookup table, to tell the fixed portion of the end-to-endconnection where the packet is going. The packet then leaves theconnection tree via output port 74.

Packet communications returning to base station 78 from outside theconnection tree will enter the connection tree at ATM Switch/VCITranslator 75 input port 73 with a virtual channel identifier of VC1.The VCI Monitor/Comparator at input port 73 reads VC1, finds row oneenabled and assigns a virtual channel identifier of VC2 to indicate toswitch 76 that the packet is destined for base station 78. The packetleaves via output port 61 and goes into input port 63 of switch 76.Switch 76 reads VC2 and finds it in the first row of its lookup table.The packet is then given virtual channel identifier VCo, is switched tooutput port 65 and is transmitted into base station 78 input port 69.

When the mobile user hands off to base station 79, it sends ATM packetswith the virtual channel identifier VC5 from output port 72. Since thelookup table for switch 76 assigns virtual channel identifier VC8 topackets which enter the switch at input port 68 with virtual channelidentifier VC5, the packets generated by the mobile user will bereceived at input port 62 of ATM Switch/VCI Translator 75 with thevirtual channel identifier VC8. Since this is not the virtual channelidentifier VC3 which was assigned by switch 76 to packets received frombase station 78, the VCI monitor/comparator at ATM Switch/VCI Translator75 realizes that a hand-off has taken place and that the mobile user iscommunicating to base station 79. Therefore, ATM Switch/VCI Translator75 updates the lookup table of the switch such that packets going fromATM Switch/VCI Translator 75 output port 61 to base station 79 inputport 71 along path 86 are switched accordingly as are packets going frombase station 79 output port 72 to ATM Switch/VCI Translator 75 inputport 62. In other words, the VCI monitor/comparator enables the secondand forth rows of the lookup table and disables the first and thirdrows.

To sum up the major features of the scheme described above: First, themobile user initiates the rerouting as the result of its hand-off bychanging the VCI of its packets. It receives all the VCIs associatedwith each base station in its connection tree at the call setup time. Bysending packets with VCIs associated to a specific base station, itinitiates the rerouting of its connections to that base station. Second,the ATM Switch/VCI Translator updates the lookup table of the switch atthe root of the connection tree so that the reverse connections areestablished accordingly. There, rerouting operations are all performedwithout the use of a centralized call processor in a distributed manner,thus enabling the system to handle frequent call hand-offs.

B. Scheme 2

In a second embodiment of the connection tree, the call set up androuting is distributed by adding one or more special ATM switches whichare called Switch/Rerouters, shown in FIG. 5. An ATM Switch/Rerouter isany switch capable of ATM Virtual Channel switching as specified byCCITT standards with an additional feature. In addition to conventionalinput ports 92 and output ports 93, it has a specific port, called theReroute Port 90, which has read/write access to the switch's routinglookup table 91.

In any mobile network with ATM packet transport, one or more ATMSwitch/Rerouters may be deployed in order to support distributed, fastconnection set up and rerouting, the details of which will be describedbelow. Again, at the connection setup time, a connection tree isestablished which connects all base stations in the neighborhood of themobile user.

Rerouting of connections as a result of hand-offs is done by sendingspecial packets called control packets to all Switch/Rerouters of theconnection tree. It is important to note that in this scheme, the mobileuser's access point will almost always see the same VCI for a mobileuser's connection. As a result, a hand-off will be totally hidden fromthe mobile user and its wireless MAC protocol.

A connection cannot, however, keep its VCI at the user-network interfaceat all times for all mobile access points. Since VCI's are reusable, aspecific user might move into an access point area in which anotherconnection has been assigned to the same VCI at the user-networkinterface. This scenario is called "terminating VCI collision." Thisproblem can be overcome in the following way. Each connection can beensured a unique VCI at the mobile access point in a given geographicalarea. Each area could cover many geographic packets. When a user movesout of the domain of its unique VCI, his or her connection is assigned anew VCI at the mobile access point interface and its adaptation layer isnotified to take actions accordingly. Also, these unique user-networkinterface virtual channel identifier domains should overlap so thatthere would be no oscillating phenomenon at their boundary. As long asthe mobile user's connection is handed off to the neighboring basestations of its connection tree, rerouting is done entirely by one ormore ATM Switch/Rerouter nodes.

Scheme 2 may be carried out with either several or only oneSwitch/Rerouter. The method employing several Switch/Rerouters withinthe connection tree will be discussed first followed by a discussion ofthe method requiring only one Switch/Rerouter at the root of theconnection tree.

Method 1: Distributed Call Setup and Reroute with SeveralSwitch/Rerouters

In FIG. 6, a simple connection tree, which has as its root ATM switch114, is shown. This connection tree includes all the neighboring basestations, each having an address, which are connected to switches 112and 113. For simplicity, only base stations 110 and 111 are shown. Allthree switching nodes 112, 113, 114 in this example are ATMSwitch/Rerouters. For specific illustration, it will be assumed that amobile user is communicating to the base station 110. The mobile user isassigned a unique Connection ID, 100, which remains the same everywherewithin the connection tree. For this case, the lookup tables at ATMSwitch/Rerouters 112 and 114 are shown in FIG. 6. Each of these ATMSwitch/Rerouter lookup tables has the capacity to have its rows eitherenabled or disabled just as in the ATM Switch/VCI Translators. In theATM Switch/Rerouters, the updating process is accomplished via controlmessages received at the reroute port, while in the ATM Switch/VCItranslators, the updating was done using a VCI Monitor/Comparator ateach input port.

In this scheme, each base station has a unique ID, distinct from theuser's connection ID. Both IDs are included in the payload of thecontrol packet, or control message. In each tree there are predefinedroutes for control messages called control virtual channel connectionswhich start from each base station and terminate at the Reroute port ofeach ATM Switch/Rerouter. The control messages are generated either bythe mobile user or by the base station accessed by the mobile user atthe time a handoff occurs. These messages are switched like any otherATM packet and arrive at the reroute ports. Upon arrival of a controlmessage at a reroute port, the ATM Switch/Rerouter identifies which rowof its lookup table should be enabled based on the Connection ID and thebase station ID contained in the payload. Where, as in our example,multiple ATM Switch/Rerouters are employed, the switch may take noaction at all.

Returning to the example, when the mobile user first communicates to theconnection tree, control signals are sent out to each of the ATMSwitch/Rerouters 112, 113, 114. The control signals enable the first andthird rows of the lookup tables for ATM Switch/Rerouters 112 and 114.This sets up connection path 120 through the connection tree. Thus, anypacket with virtual channel identifier VC1 coming into the connectiontree on input port 115 of the ATM Switch/Rerouter 114 is switched tooutput port 95 and given virtual channel identifier VC2. The packet thenenters ATM Switch/Rerouter 112 at input port 97. Since row one of thelookup table for the ATM Switch/Rerouter 112 has been enabled by thecontrol signals, the packet is assigned the virtual channel identifierVCo, which is unique to the mobile user's connection, and sent viaoutput port 101 to the input port 105 of base station 110.

In the reverse direction, an ATM packet entering ATM Switch/Rerouter 112at input port 102 from output port 106 of base station 110 with thevirtual channel identifier VC1 is transported through ATMSwitch/Rerouter 112, via output port 98 with the virtual channelidentifier VC3, into ATM Switch/Rerouter 114 input port 96 and out ofoutput port 116 of the ATM Switch/Rerouter 114 with the virtual channelidentifier VC4.

Further, for specific illustration, it will now be assumed that themobile user establishes a connection with base station 111. In thiscase, control messages are sent to the Reroute Ports of ATMSwitch/Rerouters 112, 113, and 114 by the base station 111 indicatingthe base station ID and the mobile user's connection tree ID, in thisexample 100.

Only the lookup table of ATM Switch/Rerouter 112 is changed by thecontrol messages received from base station 111 at the Reroute Port. Thecontrol message essentially tells the switch to enable connection 121and disable connection 120. To do this, the lookup table's first andthird rows are disabled and its second and forth rows are enabled. SinceSwitch/Rerouter 112 has the capability to have its lookup table updated,there is no need to update the lookup table of ATM Switch/Rerouter 114because all packets intended for the mobile user are routed fromATM/Switch Rerouter 114 to ATM Switch/Rerouter 112 whether the user isaccessing base station 110 or base station 111 and therefore only oneset of connection data need be changed.

Now, the packets entering input port 115 of ATM Switch/Rerouter 114 withvirtual channel identifier VC1 are transported through ATMSwitch/Rerouter 112, via output port 103 to input port 107 of basestation 111 with virtual channel identifier VCo along path 121. Thereverse connection along path 121 from output port 108 of base station111 to ATM Switch/Rerouter 112 input port 104 and on to output port 116of ATM Switch/Rerouter 114 is maintained as well. It is important tonote that this distributed reroute mechanism provides the same virtualchannel identifiers at mobile access point 110 or 111, as well as at ATMSwitch/Rerouter 114. As a result, the hand-off and rerouting aretransparent to the mobile user as well as to the rest of the virtualchannel/path from ATM Switch/Rerouter 114 throughout the network to theother end of the connection.

Method 2: Distributed Call Setup and Reroute with One Switch/Rerouter

In this section, it is demonstrated how the distributed call setup andreroute can be implemented with only one ATM Switch/Rerouter which isplaced at the root of the connection tree. For the purpose ofillustration, an example for this case is given using the simpleconnection tree in FIG. 7. It will be assumed that the mobile user isconnected to the network via a channel to the base station 130. In thiscase a virtual channel connection 150 is established between ATMSwitch/Rerouter 134 and base station 130. At the time the mobile userestablishes the connection, control message packets are sent to ATMSwitch/Rerouter 134 which are read by the reroute port and which enablethe first and third rows of the lookup table for ATM Switch/Rerouter134. Packets then leave base station 130 output port 144 with virtualchannel identifier VC1. The packets enter switch 151, an ordinary ATMswitch, at input port 136. The switch's lookup table directs the packetvia port 140 with virtual channel identifier VC3. The packets enter ATMSwitch/Rerouter 134 at input port 142. Since row three of the lookuptable is enabled, the packets are given virtual channel identifier VC4and are sent via port 148 to the fixed route portion of the end-to-endconnection.

Packets coming into port 147 of ATM Switch/Rerouter 134 from the fixedroute which are destined for base station 130 will have a virtualchannel identifier of VC1 and will be switched to output port 141,having been given a virtual channel identifier of VC2, as long as row 1of the lookup table is enabled. Packets then enter input port 139 of ATMswitch 132 and are switched to output port 135 with a virtual channelidentifier of VCo as indicated in row one of the lookup table. Fromthere, the packets enter base station 130 at input port 143.

When the mobile user is handed off to the base station 131, a controlmessage is sent to the ATM Switch/Rerouter 134. The Reroute Port ofSwitch/Rerouter 134 disables the first row of its lookup table andenables the second row, thus causing a new virtual channel identifier,VC7, corresponding to new base station 131, to be assigned to packetscoming into the tree from the fixed path at input port 147. Thesepackets had previously been assigned VC2, corresponding to base station130 as a destination. Now the packets enter ATM switch 132 at input port139 and their VCI corresponds to row two of the lookup table, thusresulting in their being assigned virtual channel identifier VCo andbeing sent via output port 137 to input port 145 of base station 131.

As a result, a virtual channel connection 151 will be establishedbetween ATM Switch/Rerouter 134 and base station 131 having a reverseroute from output port 146 of base station 131 to input port 138 of ATMswitch 132, via port 140, into port 142 of ATM Switch/Rerouter 134, andout of port 148. Note that the virtual channel identifier at the mobileuser's access point as well as in ATM Switch/Rerouter 134 will remainthe same as in the first method of scheme two.

One skilled in the art will appreciate that the analysis of the secondmethod of scheme two may be extrapolated to confirm that only oneSwitch/Rerouter is necessary for a more expansive connection tree if theSwitch/Rerouter is placed at the root of the connection tree. The lookuptable would, of course, be much more complex, with a greater number ofrows being needed for each connection ID.

While the above is a description of the invention in its preferredembodiment, various modifications, alternate constructions andequivalents may be employed, only some of which have been describedabove. Therefore, the above description and illustration should not betaken as limiting the scope of the invention which is defined by theappended claims.

We claim:
 1. A method of routing asynchronous transfer mode mobilecommunications comprising the steps of:receiving a packet comprising apacket header, which in turn comprises an input virtual channelidentifier for the packet, at a first input port of a multiport switch,the switch having a virtual channel identifier comparator at each inputport, each comparator having read/write access to a routing lookup tablewhich contains rows of routing information, each of the rows of routinginformation comprising an input virtual channel identifier, an inputport, an output virtual identifier and an output port; reading the inputvirtual channel identifier from the packet header; comparing the inputvirtual channel identifier read from the packet header with the inputvirtual channel identifiers contained in the routing lookup table; andenabling first and second rows of routing information within the routinglookup table, the first row of routing information comprising the outputvirtual channel identifier and output port which will be assigned to thepacket which will be routed to the destination of the mobile user'scommunication and the second row of routing information comprising theoutput virtual channel identifier and output port which will be assignedto incoming packets received at a second input port to be routed to themobile user.
 2. A method of routing asynchronous transfer mode mobilecommunications comprising the steps of:reading a control message at amultiport ATM switch having a reroute port and a routing lookup tablewhich contains rows of routing information, each of the rows of routinginformation comprising an input virtual channel identifier, an inputport, an output virtual identifier and an output port, the controlmessage comprising a mobile user's connection tree ID and base stationaddress; and enabling first and second rows of routing informationwithin the routing lookup table, the first row of routing informationcomprising the output virtual channel identifier and output port whichwill be assigned to the packet which will be routed to the destinationof the mobile user's communication and the second row of routinginformation comprising the output virtual channel identifier and outputport which will be assigned to incoming packets received at a secondinput port to be routed to the mobile user.
 3. A system for routingasynchronous transfer mode mobile communications comprising:means forreceiving a packet comprising a packet header, which in turn comprisesan input virtual channel identifier for the packet, at a first inputport of a multiport switch, the switch having a virtual channelidentifier comparator at each input port, each comparator havingread/write access to a routing lookup table which contains rows ofrouting information, each of the rows of routing information comprisingan input virtual channel identifier, an input port, an output virtualidentifier and an output port; means for reading the input virtualchannel identifier from the packet header; means for comparing the inputvirtual channel identifier read from the packet header with the inputvirtual channel identifiers contained in the routing lookup table; andmeans for enabling first and second rows of routing information withinthe routing lookup table, the first row of routing informationcomprising the output virtual channel identifier and output port whichwill be assigned to the packet which will be routed to the destinationof the mobile user's communication and the second row of routinginformation comprising the output virtual channel identifier and outputport which will be assigned to incoming packets received at a secondinput port to be routed to the mobile user.
 4. A system for routingasynchronous transfer mode mobile communications comprising:means forreading a control message at a multiport ATM switch having a rerouteport and a routing lookup table which contains rows of routinginformation, each of the rows of routing information comprising an inputvirtual channel identifier, an input port, an output virtual identifierand an output port, the control message comprising a mobile user'sconnection tree ID and base station address; and means for enablingfirst and second rows of routing information within the internal lookuptable, the first row of routing information comprising the outputvirtual channel identifier and output port which will be assigned to thepacket which will be routed to the destination of the mobile user'scommunication and the second row of routing information comprising theoutput virtual channel identifier and output port which will be assignedto incoming packets received at a second input port to be routed to themobile user.